This invention relates to an apparatus for removing ions from liquids that is suitable for use in such applications as the production of pure water in power plants (e.g. nuclear power plants), semiconductor fabrication and the production of pharmaceuticals, as well as the demineralization of concentrated liquids in the process of food and chemicals production.
There are three basic methods for removing ions from liquids, and they are reverse osmosis, electrodialysis and ion exchange. It is generally held that demineralization of seawater and other liquids of high salt concentration can advantageously be achieved by electrodialysis whereas reverse osmosis is advantageous for demineralization of liquids of lower salt concentrations. For demineralization of liquids of still lower concentrations, ion exchange is advantageous.
The conventional method of electrodialysis uses the potential difference as a drive force for transporting ions, so it has had the disadvantage that the current efficiency decreases at lower ion concentrations. Hence, the liquid treated by electrodialysis cannot be demineralized to a lower concentration than several hundred ppm. To solve this problem, a method was proposed in which the current efficiency was to be improved by packing an ion exchanger in the demineralizing compartment (U.S. Pat. No. 2,815,320 to Kollsman). This proposal was made more than 30 years ago but it has not been commercialized since it has many problems as exemplified by scale deposition on membranes and ion-exchange resins.
However, the electrically regenerable demineralizing apparatus has recently gained a new look for various reasons including the improvement in the performance of membranes, the advancement of pretreatment methods, the industrial demand for demineralizing apparatus that do not require complicated equipment for regeneration and the social demand for less consumption of resources and energy. Under the circumstances, an improved version of the early model of electrically regenerable demineralizing apparatus has been proposed (U.S. Pat. No. 4,632,745 to Millipore Corporation) and is available on the market.
The current model of electrically regenerable demineralizing apparatus has a mixture of cation- and anion-exchange resins packed in the demineralizing compartment. Ion-exchange resins are spherical beads having a diameter of 0.4-0.6 mm, so packing them uniformly in the space defined by two sheets of ion-exchange membrane and stacking a plurality of such cells to form a filter press involves a process operation that is very cumbersome and requires utmost care. If ion-exchange resins and fragments thereof leak from an end of the frame, the purity of the demineralized water will decrease. As a further problem, the liquid to be treated cannot be permitted to flow at a very high rate because the increase in the differential pressure is substantial. Some of the conventional electrically regenerable demineralizing apparatus are so adapted as to enable backwashing of contaminated or compacted ion-exchange resin layers but dispersing both cation- and anion-exchange resins uniformly is more difficult than it first appears. The most crucial reason for packing ion-exchange resins is to facilitate the transport of ions, and if the two types of ion-exchange resins separate from each other or if their presence is localized, the number of passageways available for ion transport may decrease to such an extent that it is no longer possible to attain the desired purity. Another problem with the conventional apparatus concerns the maintenance aspect and it is difficult to remove only the cell that is defective and which hence must be replaced.
Under these circumstances, it has generally been held that the electrically regenerable demineralizing apparatus is only adapted for limited applications where the capacity is small as on the laboratory scale and there are no strict requirements for the final water quality.